Abstract

Multi-user uplink transmission has been proved to be a promising technique for spectrum-efficient communications. However, due to the unique features of underwater acoustic networks (UANs), such as the long propagation delay, low transmission rate and long preamble of acoustic modems, conventional medium access control (MAC) protocols proposed for terrestrial multi-user uplink communications need an overhaul to work efficiently in UANs. In this paper, we carefully consider these features and propose a new MAC protocol, called the competitive transmission MAC (CT-MAC), for underwater multi-user uplink networks. In CT-MAC, we aim to improve the channel utilization and energy efficiency of a network by using a parallel competition mechanism. With parallel competitions, the control packets produced by each user only need to reach the direct neighbors with a low transmission power to save energy. Meanwhile, the data generated by users in different time slots can join the competition transmission in parallel to improve the channel utilization. In addition, we propose two competition schemes for CT-MAC to achieve both the short-term and the long-term fairness in different network conditions. The theoretical analysis and simulation results illustrate that compared with the conventional multi-user uplink MAC protocols, CT-MAC can achieve higher channel utilization and much lower end-to-end delay in UANs, while maintaining comparable energy efficiency.